1. Technical Field
The invention relates to a method of optimizing the release of constituent exhaust gas that has been stored in a vehicle emission control device during xe2x80x9clean-burnxe2x80x9d vehicle operation.
2. Background Art
Generally, the operation of a vehicle""s internal combustion engine produces engine exhaust that includes a variety of constituent gases, including carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). The rates at which the engine generates these constituent gases are dependent upon a variety of factors, such as engine operating speed and load, engine temperature, spark timing, and EGR. Moreover, such engines often generate increased levels of one or more constituent gases, such as NOx, when the engine is operated in a lean-burn cycle, i.e., when engine operation includes engine operating conditions characterized by a ratio of intake air to injected fuel that is greater than the stoichiometric air-fuel ratio, for example, to achieve greater vehicle fuel economy.
In order to control these vehicle tailpipe emissions, the prior art teaches vehicle exhaust treatment systems that employ one or more three-way catalysts, also referred to as emission control devices, in an exhaust passage to store and release select constituent gases, such as NOx, depending upon engine operating conditions. For example, U.S. Pat. No. 5,437,153 teaches an emission control device which stores exhaust gas NOx when the exhaust gas is lean, and releases previously-stored NOx when the exhaust gas is either stoichiometric or xe2x80x9crichxe2x80x9d of stoichiometric, i.e., when the ratio of intake air to injected fuel is at or below the stoichiometric air-fuel ratio. Such systems often employ open-loop control of device storage and release times (also respectively known as device xe2x80x9cfillxe2x80x9d and xe2x80x9cpurgexe2x80x9d times) so as to maximize the benefits of increased fuel efficiency obtained through lean engine operation without concomitantly increasing tailpipe emissions as the device becomes xe2x80x9cfilled.xe2x80x9d The timing of each purge event must be controlled so that the device does not otherwise exceed its NOx storage capacity, because NOx would then pass through the device and effect an increase in tailpipe NOx emissions. The frequency of the purge is preferably controlled to avoid the purging of only partially filled devices, due to the fuel penalty associated with the purge event""s enriched air-fuel mixture.
The prior art has recognized that the storage capacity of a given emission control device is itself a function of many variables, including device temperature, device history, sulfation level, and the presence of any thermal damage to the device. Moreover, as the device approaches its maximum capacity, the prior art teaches that the incremental rate at which the device continues to store the selected constituent gas may begin to fall. Accordingly, U.S. Pat. No. 5,437,153 teaches use of a nominal NOx-storage capacity for its disclosed device which is significantly less than the actual NOx-storage capacity of the device, to thereby provide the device with a perfect instantaneous NOx-retaining efficiency, that is, so that the device is able to store all engine-generated NOx as long as the cumulative stored NOx remains below this nominal capacity. A purge event is scheduled to rejuvenate the device whenever accumulated estimates of engine-generated NOx reach the device""s nominal capacity.
When the engine is operated using a fuel containing sulfur, sulfur is stored in the device and causes a decrease in both the device""s absolute capacity to store the selected constituent gas, and the device""s instantaneous efficiency to store the selected constituent gas. When such device sulfation exceeds a critical level, the stored SOx must be xe2x80x9cburned offxe2x80x9d or released during a regeneration or desulfation event, during which device temperatures are raised above perhaps about 650xc2x0 C. in the presence of excess HC and CO. By way of example only, U.S. Pat. No. 5,746,049 teaches a device desulfation method which includes raising the device temperature to at least 650xc2x0 C. by introducing a source of secondary air into the exhaust upstream of the NOx device when operating the engine with an enriched air-fuel mixture and relying on the resulting exothermic reaction to raise the device temperature to the desired level to purge the device of SOx.
It is an object of the invention to provide a method and system by which to control a regeneration cycle, such as a desulfation event, for an emission control device which alternatively operates to store and release a constituent gas of the exhaust gas generated by an internal combustion engine.
Under the invention, a method is provided for controlling the purging of a quantity of a constituent gas previously-stored in an emission control device of an engine exhaust treatment system, wherein the engine exhaust treatment system includes a sensor operative to generate a signal representative of the oxygen concentration of engine exhaust gas passing through the device. The method includes determining the quantity of constituent gas previously stored in the device based on the peak amplitude of the signal achieved during a first device purging; purging the device of previously-stored constituent gas at a frequency that is inversely related to the quantity of the constituent gas determined to be stored in the device; and performing a device regeneration operation to attempt to restore device capacity if the purge time is less than a predetermined minimum purge time. The method also preferably includes indicating device deterioration if a predetermined number of device regeneration operations are performed without any increase in purge time.
In accordance with another feature of the invention, the method further preferably includes producing a purge adjustment multiplier related to device capacity; and adjusting the fill time as a function of the multiplier to achieve storage of enough constituent to fill the device to a predetermined fraction of the device capacity. In an exemplary method of practicing the invention, an initial value for device fill time is determined from a lookup table as a function of an engine speed and load, for example, as an inverse power of the product of an engine load and an engine speed; or as a function of an air mass flow rate. Similarly, a default or initial value for the device capacity depletion rate is readily obtained through mapping of the engine system and the device.
From the foregoing, it will be appreciated that the invention beneficially identifies a need to regenerate the device, for example, with a desulfation event, based on the observed reduction in device storage capacity and the related increase in the storage capacity depletion rate. Thus, the device is operated continuously at its optimum condition of constituent-gas conversion efficiency, thereby minimizing tailpipe emissions while maximizing vehicle fuel economy. Intelligent regeneration of the device ensures that the constituent-gas conversion efficiency of the device is always maintained above a given minimum.
More particularly, in accordance with the invention, the device capacity depletion rate is monitored and closed-loop control of the frequency and depth of device purging, as well as closed-loop control of the desulfation of the trap, are advantageously provided. The device purge frequency is inversely related to the rate at which the selected constituent gas, such as NOx, is stored in the device, while the depth of purging is related to the quantity of the constituent gas that is subsequently released from the device during the purge event.
Furthermore, according to the invention, the device is filled to a predetermined fraction of its existing capacity based on the device capacity depletion rate, and is then completely emptied during a purge. As the device capacity decreases, for example, due to device component deterioration, a closed-loop purge optimization routine produces an adjustment multiplier that is used to adjust the device capacity depletion rate in order to achieve constituent gas storage that is just enough to fill the device to the desired fraction of its capacity. As the device capacity is substantially reduced, as indicated by the actual device capacity depletion rate becoming equal to or greater than a predetermined maximum capacity depletion rate, a device regeneration event is scheduled with a view toward restoring lost device capacity. If a predetermined number of device regeneration operations are performed without any significant increase in device capacity, the device must be replaced and the operator is so informed by an indicator.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.